1. Field of the Invention
The present invention generally relates to radio communication techniques, and more particularly, to a method for transmitting Hybrid Automatic Retransmission reQuest-ACKnowledgement (HARQ-ACK) information based on transmission diversity.
2. Description of the Related Art
Existing Long Term Evolution (LTE) systems have a maximum bandwidth of 20 MHz which cannot meet the requirement of higher data rates. Currently, in order to increase transmission rates of users, LTE-Advanced (LTE-A) has been proposed, based on LTE. In LTE-A systems, multiple Component Carriers (CCs) are combined to provide a wide working bandwidth and form a downlink and uplink of a communication system, so as to provide higher transmission rates. This technique is referred to as a Carrier Aggregation (CA) technique. For example, in order to support 100 MHz bandwidth, five CCs of 20 MHz may be combined. Hereinafter, each CC is referred to as a Cell.
Among multiple downlink Cells configured by a base station, one is a Primary Cell (Pcell) and others are Secondary Cells (Scells). The base station configures the UE to receive downlink data from multiple CCs through higher layer signaling. The number of Cells actually scheduled in one subframe may be less than or equal to the number of Cells configured by the higher layer. For example, in FIG. 1, two Cells are configured by the higher layer, respectively, Cell 1 and Cell 2, whereas the base station actually schedules one Cell, i.e. Cell 1.
The data transmission in one downlink Cell may be scheduled by a Physical Downlink Control CHannel (PDCCH) transmitted in another Cell, which is referred to as cross-carrier scheduling. The data transmission in one downlink Cell may also be scheduled by the PDCCH transmitted in the same Cell, which is referred to as non-cross-carrier scheduling, as shown in FIG. 2.
Based on the CA technique, the base station transmits downlink data to the same UE on multiple Cells. Accordingly, the UE needs to return HARQ-ACK information in response to the downlink data transmitted on the multiple Cells. According to current discussions involving LTE-A, the HARQ-ACK information of the downlink data of Cells within one CA is transmitted in one uplink Cell (i.e. uplink Pcell). In order to support the transmission of HARQ-ACK information of multiple bits, at most 4 bits of HARQ-ACK information may be transmitted in LTE-A adopting a channel selection based method, which has been used in LTE Time Division Duplexing (TDD) systems. When the channel selection of a single antenna is considered, the number of bits of HARQ-ACK information to be allocated is equal to the number of the HARQ-ACK bits. For simplicity, suppose there are M bits of HARQ-ACK information, and M HARQ-ACK channels are allocated accordingly. Since each HARQ-ACK channel has four available Quadrature Phase Shift Keying (QPSK) constellations, 4M channel and constellation resources are obtained. Appropriate channel and constellation resources may be selected from the 4M resources for returning the M bits information.
According to the current discussions regarding LTE-A, in LTE-A Frequency Division Duplexing (FDD) systems, the channel selection method supports only aggregation of two Cells and each Cell may return 1-bit or 2-bit HARQ-ACK information. The method for allocating the HARQ-ACK channel resource is as follows.
For a downlink Pcell, the HARQ-ACK channel used by the HARQ-ACK information of the Pcell is determined according to a Control Channel Element (CCE) index of the PDCCH via a connotative method.
For a downlink Scell, if the Scell is not cross-carrier scheduled from the PDCCH of the Pcell, the HARQ-ACK channel used by the HARQ-ACK information of the Scell is determined according to a HARQ-ACK Resource Information (ARI) in the PDCCH of the Scell; if the Scell is cross-carrier scheduled from the PDCCH of the Pcell, the HARQ-ACK channel used by the HARQ-ACK information of the Scell is determined via a connotative method according to the CCE index of the PDCCH.
If the Cell is configured with a Single Input Multiple Output (SIMO) transmission mode, since it is only required to return one HARQ-ACK with respect to one Transmission Block (TB) of the Cell, one HARQ-ACK channel needs to be allocated. Accordingly, if the Cell is configured with a Multiple Input Multiple Output (MIMO) transmission mode, two HARQ-ACK need to be returned with respect to two TBs of the Cell. Therefore, two HARQ-ACK channels need to be allocated. As to the situation of allocating HARQ-ACK channels via the connotative method, the HARQ-ACK channel used by the HARQ-ACK information of one Cell is obtained through the PDCCH scheduling the data transmission of the Cell. In particular, the index of a first CCE of the PDCCH is denoted as nCCE. If one HARQ-ACK channel needs to be allocated, the HARQ-ACK channel may be mapped according to the index nCCE of the first CCE. If two HARQ-ACK channels need to be allocated, the two HARQ-ACK channels may be mapped according to the first CCE index nCCE and the second CCE index nCCE+1.
According to the current discussions regarding LTE or LTE-A Release 10 (Rel-10), if two Cells are both configured with the MIMO transmission mode, four HARQ-ACK bits will be generated. If the Physical Uplink Control CHannel (PUCCH) format 1b with channel selection is adopted for transmission, four PUCCH resources are required. If a Spatial Orthogonal Resource Transmission Diversity (SORTD) method is adopted, eight PUCCH resources are required. According to the discussions regarding LTE-A, the number of PUCCH resources does not exceed 4 when transmission diversity is adopted.
In order to solve the above problem, one current method is as follows: if the Cell is configured with the MIMO transmission mode, two HARQ-ACK bits of each CC is converted into one bit through spatial bundling. Thus, two HARQ-ACK bits are generated for 2 CCs. Then, the two bits are mapped according to Table 1.
TABLE 1HARQ-ACK(0)HARQ-ACK(1)nPUCCH,i(1)b(0)b(1)ACKACKnPUCCH,1(1)1, 1ACKNACK/DTXnPUCCH,0(1)1, 1NACK/DTXACKnPUCCH,1(1)0, 0NACKNACK/DTXnPUCCH,0(1)0, 0DTXNACK/DTXNo transmission
In Table 1:
“ACK” denotes that the content of HARQ-ACK information directly generated by the UE for a Cell or generated through spatial bundling for the Cell is positive acknowledgement;
“NACK” denotes that the content of HARQ-ACK information directly generated by the UE for a Cell or generated through spatial bundling for the Cell is negative acknowledgement;
“DTX” denotes that the UE does not receive data on the Cell;
nPUCCH,i(1) denotes the channel used for transmitting the HARQ-ACK information; and
b(0)b(1) denotes the HARQ-ACK bits transmitted. HARQ-ACK (0) is HARQ-ACK information from the Pcell, and HARQ-ACK(1) is HARQ-ACK information from the Scell.
When being transmitted on antenna 0, nPUCCH,0(1) is a channel mapped by the index nCCE of the first CCE of the PDCCH scheduling the Physical Downlink Shared CHannel (PDSCH) of the PCell via the connotative method, nPUCCH,1(1) is a channel mapped by the index nCCE of the first CCE of the PDCCH scheduling the PDSCH of the Scell or the first channel indicated by the ARI in the PDCCH scheduling the PDSCH of the Scell.
When being transmitting on antenna 1, nPUCCH,0(1) is a channel mapped by the index NCCE of the second CCE of the PDCCH scheduling the PDSCH of the Pcell, nPUCCH,1(1) is a channel mapped by the index nCCE of the second CCE of the PDCCH scheduling the PDSCH of the Scell or the second channel indicated by the ARI in the PDCCH scheduling the PDSCH of the Scell.
Information transmitted on antenna 0 and antenna 1 is the same.
According to the current discussions regarding LTE-A, in LTE-A TDD systems, if the HARQ-ACK bundling window is 1, the same method as the FDD is adopted, and the HARQ-ACK information are mapped according to the method shown in Table 2.
TABLE 2HARQ-ACK(0), HARQ-ACK(1)nPUCCH(1)b(0)b(1)ACK, ACKnPUCCH,1(1)1, 0ACK, NACK/DTXnPUCCH,0(1)1, 1NACK/DTX, ACKnPUCCH,1(1)0, 1NACK, NACK/DTXnPUCCH,0(1)0, 0DTX, NACK/DTXNo transmission